Bone · Head

Ethmoid

Os ethmoidale

A light, spongy block of bone set between the two eye sockets, doing four jobs at once: it roofs the nasal cavity, forms the upper part of the septum dividing the nostrils, supplies the paper-thin inner wall of each orbit, and provides two of the three scrolls on the side wall of the nose. It is riddled with air cells — the ethmoid sinuses — which is why so large a bone weighs so little. Its roof, the cribriform plate, is perforated like a sieve, and the fine filaments of the olfactory nerve pass up through those holes from the lining of the nose to the olfactory bulb sitting on top of it. Nothing about the ethmoid can be touched or tested from outside; it is understood by knowing what it separates.